Applications of satellite navigation systems such as United States' GPS, Russia's GLONASS, Europe's Galileo, and China's Beidou are increasingly pervasive. Although GNSS technology is relatively reliable, system faults do occur. On average, about one major satellite fault occurs per year. The most common satellite faults involve a clock failure, in which an atomic clock on a satellite suddenly diverges from the time standard for the rest of the satellite constellation. When a clock failure occurs, the ground segment responds by temporarily flagging a particular satellite as unhealthy, reconfiguring the satellite to use a backup clock, testing the satellite to verify the performance of the backup clock, and flagging the satellite as healthy again at the conclusion of this testing period. This process ensures the overall health of the GNSS satellite constellation in a manner that is satisfactory for many users. However, the status indicator for a GNSS satellite may remain “healthy” for several minutes or tens of minutes after a fault has occurred.
For safety-of-life navigation applications, even a brief exposure to a significant fault like a clock failure can compromise system integrity. Satellite failures present a serious risk because they cannot be detected by the GNSS ground segment immediately. During the time after a satellite fault first occurs, positioning errors may grow steadily, eventually reaching levels of tens or even hundreds of meters. These errors are large enough to create a significant risk of collision for precision flight and driving applications.
Several technologies have been developed to detect anomalies quickly. Space-based augmentation systems (SBAS), such as the Wide Area Augmentation System (WAAS) in North America and the European Geostationary Overlay Navigation Service (EGNOS) in Europe, rely on a continent-scale network of GNSS receivers which collect data that is compiled by a master station. The master station transmits the SBAS data to a satellite in geostationary orbit, which broadcasts the data to users over a wide area. These systems are highly sensitive, but they are only available in certain areas of the world. Also, they use an indirect communications path which creates a delay of about 12 seconds from the time of fault until an alert reaches a user. For high precision operations, more rapid response times are necessary.
Another integrity monitoring technology is Receiver Autonomous Integrity Monitoring (RAIM), which provides a much faster response but a much lower sensitivity than SBAS. In RAIM, a GNSS receiver estimates its position and computes the estimation residuals (e.g., the inconsistency between each satellite ranging measurement and the final position estimate). Large residuals may be indicative of a fault condition. Because RAIM relies only on local GNSS measurements, there is not a large communications delay that causes a lag in the time-to-alert. However, RAIM has poor sensitivity, as measurements are produced by only a single receiver that typically is of moderate quality. RAIM is most useful for coarse navigation applications, but with precision of about 50 meters, it is not sensitive enough for precision navigation applications.
Ground-Based Augmentation Systems (GBAS) can achieve both high sensitivity and fast response. GBAS uses a network of high quality receivers at fixed locations in a local area, such as around an airport facility, to detect anomalies. Warnings are generated by a central processor and transmitted to users in the local area. This configuration provides very sensitive integrity monitoring, with an alert time as low as 2 to 6 seconds. However, a GBAS system supports users only over a very localized area, e.g., tens of kilometers, so a large number of GBAS installations would be required to support a geographically large area.